This invention relates to electromagnetically operated display or indicating elements wherein a rotatably mounted disc contrastingly coloured on opposite sides is controlled by a reversible, exterior magnetic field, created by one or two electromagnetically energized poles or "pole pieces" which controls a magnet mounted on and rotatable with the disc to cause the disc to display one or the other of its contrasting faces in a viewing direction. Such discs may be used singly as indicators or may be used in multiple arrays to collectively convey information to a viewer.
Examples of such arrangements are shown in U.S. Pat. Nos.
3,295,238; Jan. 3, 1967; D. Winrow PA1 3,303,494; Feb. 7, 1967; M. K. Taylor et al PA1 3,385,824; Jan. 30, 1968; D. Winrow PA1 3,871,945; Mar. 18, 1975; D. Winrow et al PA1 3,996,680; Dec. 14, 1967; C. N. Smith PA1 3,156,872; May 29, 1979; G. Helwig
and all assigned to the applicant herein.
In some variations of such arrangements, the rotation of the disc, on reversal of the exterior field has been limited to a small amount less than 180.degree. by a mechanical stop or stops, which are arranged relative to the exterior field so that on each rotation of the disc it is mechanically stopped short, by a small angle, of the position where the permanent magnet aligns with the exterior field. As a result, when the exterior field is reversed, the rotary disc and magnet are impelled in only one direction by the exterior field and one major cause of mal-operation or "hang ups" was avoided. Such an arrangement is exemplified by U.S. Pat. No. 3,295,238 referred to above. The use of such mechanical stopping although it has removed a cause of operating failure, has introduced a further cause of such failure. In some cases, particularly under conditions of high humidity, there has been a tendency of the rotating disc to adhere, after a period of rest to the mechanical stop, causing it, where such adherence occurs, to refuse to move when the exterior field is reversed to cause such movement. Reduction of the occurrence of this adherence has hitherto been achieved only by expensive cleaning and neutralizing operations to remove foreign films acquired during normal manufacturing processes.